A capacitance detecting proximity sensor detects the proximity of an object (detection subject) such as a human body by measuring the change in capacitance arising due to that object. The principle of such detection is simple and has been known for a long time, but improvements for raising the precision, reliability, and ease-of-use of the detection continue even now, and various configurations have been proposed. The configurations of capacitance detecting proximity sensors can be broadly divided into two formats.
The first format detects/measures, with an appropriate method, the capacitance (capacitance to ground) that one detection electrode disposed facing an open space forms with respect to a common ground potential (or common reference potential). When a detection subject approaches the detection electrode, the capacitance to ground changes due to the affect of the detection subject. This change is measured and the proximity of the detection subject is detected. See for example Japanese Patent Application Laid-Open Publication (JP-A) No. 2001-35327.
The second format measures the capacitance (inter-electrode capacitance) formed between two mutually opposed detection electrodes. When a detection subject comes between the two electrodes, the inter-electrode capacitance changes due to the affect of the detection subject. This change is measured and the proximity of the detection subject is detected. See for example JP-A No. 2001-26446.
It has been made apparent by the present inventor that there are the following problems in the aforementioned techniques.
Namely, in the first format, by disposing the detection electrode facing the open space, the proximity of the detection subject from the open space can be detected. In other words, the detection range can be set to a spatially open region. Thus, an optional object such as human body or a fixed object, for example, can be made to serve as the detection target.
However, the capacitance to ground of the detection electrode disposed facing the open space is easily affected by the environment or certain peripheral objects outside of the detection target. When an object forming a large capacitance is present in the vicinity of the proximity sensor, this triggers a detection malfunction even if the object is at a non-proximate position outside of the detection target.
For example, in order to automatically prevent fingers from being pinched in a sliding or hinged automatic open/close door attached to a vehicle such as a wagon, whether or not a person's fingers are in a position where there is the danger of them being pinched is detected by measuring the change in the capacitance to ground, and the drive operation for closing the door is controlled on the basis of this detection. However, this detection is easily affected by the car body and the door and can easily malfunction. Because the car body and the door form a large capacitance between them and the detection electrode, it is easy for them to be erroneously detected even if they are in a non-proximate position. Even airborne matter such as raindrops and mist can trigger a detection malfunction.
In the second format, because the detection range can be substantially limited to the closed space between the two detection electrodes, the affects of peripheral objects that are present outside of the closed space can be reduced. However, objects that cannot enter this closed space cannot be made to serve as the detection targets. Optional objects such as human bodies or fixed objects cannot be made to serve as the detection targets. Moreover, even in the second format, airborne matter such as raindrops and mist can trigger a detection malfunction.
By disposing two detection electrodes on the same surface, the detection range can be formed in a spatially open region in the same manner as in the first format, but in this case the same problems as in the first format arise. See for example JP-A No. 2000-48964.
In this manner, although capacitance detecting proximity sensors have the advantages that their principle is simple and they can be configured relatively easily, it is easy for malfunctions resulting from the affects of peripheral objects outside the detection target to arise, and there have been problems in the precision, reliability, and ease-of-use of the detection. For this reason, there have been many inadequacies in using the sensors to prevent fingers from being pinched in automatic open/close doors in automobiles, and fundamental improvements have been needed.